Recently, the transferring efficiency of data is improved by various methods, with scale expansion, the diversification and the greater complexity of channels. Under such circumstances, with a conventional system like the one disclosed in International Publication Pamphlet No. WO2002/097476, in an optical network, downstream and upstream data are transmitted in variable-length packets utilizing time division multiplexing with variable-length time slots. The length of the variable-length time slots changes in response to the upstream traffic demand.
However, with a conventional system using the variable-length time slots, it is not possible to communicate by frames, such as in Ethernet (registered trademark) and the like, and the transferring efficiency of frames could not be improved. It is desirable to improve the transferring efficiency of frames, especially for Ethernet (registered trademark), the most widely used communication system of LAN (Local Area Network).